In a dramatic reversal of decades of public opinion, California voters agree by a slim majority that same-sex couples should be allowed to marry, according to a Field Poll released today.

By 51-42 percent, registered voters said they believed same-sex marriage should be legal in California. Only 28 percent favored gay marriage in 1977, when the Field Poll first asked that question, said Mark DiCamillo, the poll's director.

"This is a milestone in California," he said. "You can't downplay the importance of a change in an issue we've been tracking for 30 years."

While opposition to same-sex marriage has been weakening for years in California, supporters have remained a minority. In March 2000, for example, voters overwhelmingly backed Proposition 22, a statute that said the state would recognize only the marriage of a man and a woman. A 2006 Field Poll showed that half the state's voters still disapproved of same-sex marriage.

But the state Supreme Court's decision this month to overturn Prop. 22 might have turned the tide, DiCamillo said.

"There's a certain validation when the state Supreme Court makes a ruling that you can't discriminate when it comes to marriage," he said. "That may have been enough to move some people who were on the fence about same-sex marriage."

Younger voters and those living in the Bay Area, Los Angeles and other Democratic urban strongholds were the most supportive of same-sex marriage, the poll found, while older voters and those living in the more conservative inland areas were more opposed.

The poll also provided a boost for groups planning to battle a measure to ban same-sex marriage that is expected to go on the ballot in November as a constitutional amendment. By 51-43 percent, registered voters oppose changing the state Constitution to ban same-sex marriage, according to the poll.

Results differ from other poll

A statewide Los Angeles Times/KTLA Poll released last week showed different results: 54 percent of registered voters said they would support the initiative that would change the state Constitution to ban same-sex marriage.

"When we get results that we think are surprising, we double- and triple-check our numbers, and that's what we did here," said DiCamillo. "Everything in this poll is consistent internally."

It's not unusual for two polls to have conflicting numbers, said Steve Kinney, a veteran GOP pollster.

"It's all in the methodology, who you actually talked to and whether they accurately represented that state as a whole," he said. "But even if you have confidence in your numbers, you're always scared if you come up with something totally different. Are you wrong, or is the other guy?"

Support for same-sex marriage has been growing steadily in California, and the youngest voters are pushing the hardest. Among voters 18 to 29 years old, 68 percent back gay marriage, compared with only 36 percent of those 65 and older, the Field Poll found.

It's a "generational replacement, with older folks being replaced by younger voters very much in favor of same-sex marriage," DiCamillo said.

Those younger voters "have grown up with people who are out in their lives, whether it's politicians in the news or people they know," said Geoff Kors, executive director of Equality California, one of the groups opposed to the proposed initiative to ban same-sex marriage.

But the dramatic movement on the issue over the past few years hasn't been by accident, Kors said. San Francisco Mayor Gavin Newsom's dramatic - and later overturned - decision to issue marriage licenses to gay couples in 2004, the Legislature's passage of two bills to authorize same-sex marriage and other efforts have helped educate people about the issue and bring same-sex marriage into the California mainstream, he said.

"Legislators voted for same-sex marriage, and none of them were voted out of office," Kors said. "This (poll result) is what we'd expect, but it's also the first time we've seen a majority for same-sex marriage, and the Field Poll is as credible as it comes."

Supporters of the new initiative shrugged off the poll results, saying that it is a long time until the November election.

"The Supreme Court ruling only just happened," said Karen England, spokeswoman for the Capitol Resource Institute, one of the groups backing a ban on same-sex marriage, which is expected to be approved for the ballot in mid-June. "Once we have the measure on the ballot, the campaign can change everything."

England said plenty of voters calling her group are outraged that the Supreme Court overturned Prop. 22 after it passed with 61 percent of the vote. She also questioned the accuracy of pre-election polls, noting that support for Prop. 22 on election day was much stronger than was shown in polls taken days before the vote.

"People might say one thing to a pollster, but they make their own decision in the voting booth," she said.

How state splits on issue

The new Field Poll highlights the battleground for the fall campaign, showing the state splitting dramatically along regional, ideological and religious grounds.

The heavily Democratic urban areas strongly support same-sex marriage; 55 percent of Los Angeles County and an overwhelming 68 percent of the Bay Area are in favor. By contrast, only 38 percent of the Central Valley and 41 percent of Southern California outside of Los Angeles are in favor.

Same-sex marriage also digs a chasm between California's heavily populated coast and its inland areas; 55 percent of coastal voters back same-sex marriage compared with 40 percent in support inland.

The issue divides along liberal-conservative lines; 85 percent of strong liberals are in favor, and 85 percent of strong conservatives are opposed.

Protestants, who make up a third of the state's voters, oppose same-sex marriage 34 percent to 57 percent, while Catholics are split almost equally, 45 percent in favor to 48 percent opposed. Those with no religious preference back same-sex marriage 81 percent to 12 percent.

America will be watching the fall campaign over same-sex marriage, which will have national implications, said Kors of Equality California.

"It's going to be an intense and enormous undertaking, but we're confident we'll win," he said. "But we also know that the other side is out there working just as hard and feeling just as confident."

The poll is based on a telephone survey of 1,052 registered voters taken May 17-26. The margin of error is plus or minus 3.2 percentage points.

Status report

The law: The state Supreme Court this month overturned a ban on same-sex marriage approved by voters in 2000.

Weddings: Counties are preparing to issue marriage licenses to same-sex couples when the ruling takes effect in mid-June.

New initiative: A constitutional amendment declaring that "only marriage between a man and a woman is valid" is likely to qualify for the November ballot.